The fabrication of very large-scale integrated circuits requires processes that are compatible with small feature sizes (e.g., 0.25 μm). A particular problem is the etching of a silicon wafer or other substrate assembly to produce damascene layers, self-aligned contacts (SACs), or trench isolation. These features typically require etching relatively deeply into the wafer while maintaining a small footprint on the surface of the substrate assembly, i.e., these features have a high-aspect-ratio (HAR), with a depth-to-width (on the surface of the substrate assembly) ratio of 4:1 or larger.
Features to be etched into a substrate assembly are typically defined with a layer of photoresist that is spin-coated or otherwise applied onto a surface of the substrate assembly and then photolithographically patterned. After patterning, some areas of the substrate assembly surface remain covered by the photoresist layer while other areas are exposed. The covered substrate assembly is exposed to an etch, and the photoresist layer prevents etching except in the exposed areas.
Etching of HAR features requires anisotropic etches that etch more rapidly in one direction than another. Conventional wet etches include dilute solutions of acids such as hydrofluoric acid. While wet etching is simple and inexpensive, wet etching is generally inadequate to produce HAR features because wet etches tend to etch isotropically. In addition, it is difficult to etch deep HAR features into a substrate assembly because the etchant does not flow freely into and out of the feature. Therefore, even if a wet etch begins to etch properly, etchant is consumed within the feature being etched and is replenished slowly.
Dry etching with plasmas is also used for etching substrate assemblies. In plasma etching, a gas or gas mixture is fragmented and ionized and the ions produced are accelerated toward the substrate assembly. When the ions reach the substrate assembly, they combine chemically with the substrate assembly to form volatile compounds that are readily driven off of the substrate assembly. In some cases, the mechanical impact of the ions with the substrate assembly also serves to etch the substrate. Because of the acceleration of the ions toward the substrate assembly, etching is anisotropic and proceeds rapidly on surfaces that are perpendicular to the propagation direction of the ions.
Unfortunately, dry etching with a plasma has significant limitations. While plasmas etch anisotropically, a plasma etches both the substrate assembly and the photoresist that defines the features to be produced. As a result, the total etching time is limited by the time required for the plasma etch to penetrate the photoresist. When the photoresist is penetrated, further etching is no longer limited to the intended substrate locations, but occurs in all substrate areas that are not protected by the photoresist. Photoresists typically etch four to five times more slowly than typical substrate materials to be etched (such as silicon or silicon oxide). Etching processes in which a substrate material is etched at a rate of less than about eight times the rate at which a resist etches are referred to herein as “resist-consuming.”
Etching deep HAR features requires thick layers of photoresist to permit long etch times, and such thick layers complicate the photolithographic patterning process. For example, to etch a HAR feature 3000 nm deep requires a photoresist thickness of as much 750 nm. Patterning a feature as small as about 250 nm is very difficult in such a thick layer of photoresist.
Other factors limiting plasma etching include the difficulty of providing a selected distribution of ions (charged particles) and neutral particles at the substrate surface and at the bottom of a feature being etched. Accordingly, improved etching methods are needed, especially for etching high-aspect-ratio features. A resist layer has a nominal thickness and a facet thickness, either or both of which are maintained, preserved, or increased in the disclosed methods and apparatus.